1. Field of the Invention
The claimed invention relates to digital broadcast devices, and more particularly, to multi-system digital broadcast signal receiving devices.
2. Description of the Prior Art
Digital broadcast signals can be categorized into digital audio broadcasting (DAB) signals and digital video broadcasting (DVB) signals, where the DAB signals correspond to various standards such as Eureka-147 in Europe (also adopted by Taiwan), IBOC in the USA and DRM in France, and the DVB signals correspond to various standards such as DVB-T and DVB-H, whose signals have different bandwidths, for example, the bandwidth of DVB is 6, 7, or 8 MHz, and the bandwidth of DAB is 1.536 MHz. In addition, some countries also develop other standards such as T-DMB (e.g. a Korean mobile TV standard). Therefore, to receive these multi-system signals, the receiving systems have to be provided with special design.
In order to integrate multi-system signal receiving system into a single receiver, sharing a common tuner seems to be a feasible way that may accomplish the purpose. Since surface acoustic wave (SAW) filters are used as channel selection filters within tuners, digital filters can be utilized for selecting channels regarding standard(s) with a narrower signal bandwidth (such as DAB) in order to prevent from using SAW filters of various bandwidths and hence to prevent from raising the corresponding cost. FIG. 1 illustrates a conventional receiving system for the DVB-T and DAB standards. Please refer to FIG. 1. The receiving system 100 comprises a tuner 101, an analog-to-digital converter (ADC) 103, a down converter 105, a digital low pass filter 107, a fast Fourier transform (FFT) circuit 109, a back-end processing circuit 111 and a synchronization circuit 113, where the synchronization circuit 113 is utilized for providing the FFT circuit 109 with synchronization information. Detailed structure and operations of the receiving system 100 according to the prior art are well known by those skilled in the art, and therefore, are omitted here for brevity.
Please note that the conventional receiving system 100 adopts a high order digital low pass filter 107 to correctly receive signals. Since the digital low pass filter 107 has to filter DAB signals out and the bandwidth of the guard band between DAB channels is about only 176 KHz, a high order digital low pass filter is required. FIG. 2 illustrates the frequency response of the digital low pass filter shown in FIG. 1, where the bold line portion represents the frequency response of the digital low pass filter 107 having its pass-band frequency and stop-band frequency being 768 KHz and 944 KHz, respectively. If the conventional receiving system 100 uses the lower order digital low pass filter 107, the synchronization circuit 113 may be unable to precisely detect DAB frames due to adjacent channel signal (ACS) interference, and therefore, be unable to output correct synchronization signals. As a result, the FFT circuit is incapable of correctly performing FFT operations.
As the conventional architecture is hard to prevent from utilizing a circuit having higher cost, such as the high order digital low pass filter, a novel invention is required for solving the problems mentioned above.